1. Field of the Invention
The present invention relates to an article (substrate, product) made of glass-ceramic, in particular a glass-ceramic plate, intended, for example, for covering or accommodating heating elements, such as for example a hob, an oven door, or a chimney insert, or a fire screen, etc., to a process for obtaining said article, and to a novel enamel composition suitable for the coating thereof. More particularly, the present invention relates to a mechanically reinforced glass-ceramic article and also to the process of reinforcing a glass-ceramic article and/or to the enamel that makes it possible to obtain said reinforced glass-ceramic article.
Sales of articles such as glass-ceramic hobs have been continuing to grow over the last few years. This success is explained in particular by the attractive appearance of such hobs and by the ease of cleaning them.
It will be recalled that a glass-ceramic is originally a glass, called precursor glass (or mother glass. or green glass), the specific chemical composition of which allows controlled crystallization to be induced by suitable heat treatments, called ceramization. This partly crystallized specific structure gives the glass-ceramic unique properties.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
At the present time, there are various types of glass-ceramic plate, each variant being the result of extensive research and many tests, given that it is very difficult to make modifications to these plates and/or to the process for obtaining them without risking an unfavorable effect on the desired properties. In particular, to be able to be used as a hob, a glass-ceramic plate must generally have a transmission in the wavelengths of the visible range that is both low enough to mask at least some of the subjacent heating elements when not in use and high enough so that, depending on the case (radiant heating, induction heating, etc.), the user can, for the sake of safety, visually detect the heating elements when they are in operation and/or can, where appropriate, read the displays. It must also have a high transmission in the wavelengths of the infrared range, especially in the case of hobs with radiant burners.
The glass-ceramic plates must also have a sufficient mechanical strength as demanded in their field of use (for example, in accordance with the EN 60335-2-6 standard for hobs in the field of household electrical goods). In particular, in order to be able to be used as hobs, the glass-ceramic plates must have sufficient resistance to the pressure and to the shocks that may arise (support and dropping of utensils, etc.). Generally, the glass-ceramic plates alone have a mechanical strength that is expressed in particular by a scale factor (defined below) between 150 and 180 MPa.
Most current plates are of dark color, in particular black, but there are also plates of lighter color (in particular white having, for example, a haze of at least 50%, as described in patent FR 2 766 816), or even transparent plates provided with opacifying coatings. Among known (functional and/or decorative) coatings for glass-ceramic plates, there are conventionally enamels, based on glass frits and pigments, and certain paints resistant to high temperature, based for example on alkyd resins. In particular, enamels have the advantage of being able to be deposited on the precursor glass (or mother glass or green glass) before ceramization and of being able to be baked during the ceramization, and also have the advantage of being able to withstand high temperatures (allowing the use of various heating means for the plate). However, they have the drawback of generally permitting only a single deposition (no enamel superposition is possible) and with a small thickness, otherwise there is a risk in particular of the enamel flaking off and of mechanically damaging the glass-ceramic plate. As regards paint, this may be applied (if so required) as several layers. However, it must be applied after ceramization (and therefore requires an additional baking operation) and remains limited to plates for induction burners (operating at lower temperature).
More recently, glass-ceramic plates have also been proposed with coatings based on reflective layers deposited by magnetron sputtering or based on glass batch materials incorporating special-effect pigments (aluminum oxide or mica flakes coated with metal oxides). However, the coatings based on layers deposited by magnetron sputtering are more expensive since they require a specific installation and are generally limited to plates for induction burners, and their manufacture, carried out after ceramization, is more complex or tricky. As regards coatings based on a glass batch with special effect pigments, they have the same drawbacks as the abovementioned enamels.